It's a very simple application: you can drag and drop an image of your choice to the tool, and at that point you can choose from compression criteria including Google's MozJPEG, OptiPNG, and WebP.

The small slider lets you set the compression level quickly, and you'll see a total of the expected amount of file size.

A second slider can be dragged across the image to let you easily see the effects of compression.

There are more advanced options available, but we were able to find a decent balance between image size and quality without it.

Interestingly, the Chromium blog announcing the gadget notes that its original goal was to show the speed with which web apps can load in Chrome, even if they have to do heavy work like drawing on the Web Assembly to take advantage of out-of-browser codecs.